The present invention relates to semiconductor integrated circuit fabrication. More particularly, the present invention provides a technique, including a method and structure, for high resolution photolithography of reflective layers.
Photolithography is a common process used in the fabrication of integrated circuits or integrated circuit devices. These devices are made on a semiconductor wafer using various process steps. These process steps are performed on the wafer to form integrated circuit elements using, for example, patterns on thin layers, e.g., insulating material, metal, silicon, etc. Photolithography techniques create these patterns on the thin layers.
Generally, these photolithographic techniques are a sequential series of steps used to produce the patterns on the thin layers. These steps include applying a layer of photosensitive material, often termed photoresist, to the surface of the wafer, and exposing portions of the photoresist through a patterned mask or photomask. The photomask is typically a glass plate having a pattern, like a picture, on one side. The patterned side is placed next to, and sometimes on, the photoresist layer during exposure to insure the accurate transfer of the pattern from the mask to the photoresist layer.
A special lamp is used to expose the photoresist layer. Typically, this lamp produces light of a predominant single wavelength. The exposure is timed so that the desired amount of light shines on the photoresist. Too much or too little light can result in a poor exposure. The lamp is also designed to shine the light perpendicular to the mask to insure the accurate reproduction of the pattern on the mask to the photoresist. Any light incident to the photoresist layer at an angle can expose photoresist under the shadowed area of the patterned mask where exposure is undesirable.
After exposure, the photoresist is developed. Typically, the wafer is placed in a solvent bath, washing away the undesired photoresist. If the photoresist is a positive type, the photoresist that was under the shadowed areas of the mask remains, and the areas of resist that were exposed to the light wash away. This leaves a pattern in the photoresist that is similar to the pattern on the mask, in other words, a positive image of the mask pattern. If the photoresist is a negative type, the areas that were exposed to the light remain, and the areas that were shadowed wash away. This leaves a pattern of photoresist that is a negative of the pattern on the mask.
The pattern of photoresist can now be transferred to the underlying layer on the wafer by etching or other processes. For example, the wafer can be placed in a chemical bath that dissolves exposed portions of the layer where the layer is not protected by the photoresist. The goal is to obtain a pattern on the wafer that is as faithful as possible to the pattern on the mask.
It is often difficult to obtain a faithful pattern on highly reflective layers because the amount of light that the layer reflects can partially expose some of the photoresist. Highly reflective layers include layers of metals, such as aluminum or gold, semiconductors, such as silicon, or silicides, which are alloys of silicon and metal. This problem becomes more significant as the patterns become finer because the amount of reflected light has a greater relative effect on exposing a thin line, which casts a narrow shadow, than a thick line, which casts a wide shadow. An antireflection coating on top of a highly reflective layer can improve the resolution of the photoresist pattern and the subsequent precision of the pattern in the reflective layer.
Various methods have been proposed to produce an antireflection coating on a reflective surface in order to improve photolithographic processing. The use of a polyimide layer with a light absorbing dye is known. This method, however, requires several critical process steps which are difficult to control. Additionally, the polyimide coating must be thin in order to preserve the precision of the exposed pattern. However, a thinner layer absorbs less light. Hence, the polyimide layer thickness is generally a compromise between good light absorption and good pattern definition. This polyimide layer is also typically removed before more wafer processing occurs.
Another approach is to use a coating of titanium compounds, such as titanium nitride (TiN) or titanium dioxide (TiO.sub.2) on top of a metal layer to suppress standing waves. These approaches rely on precisely matching the thickness of the coating to the wavelength of light used in the exposure of the photoresist. Not only is the precise control of the deposition of the dielectric layer difficult, producing such an antireflection layer must intend exposure of the photoresist with a particular wavelength of light. Substrates coated to be used with one light source will perform poorly if exposed with another.
An additional problem with this method is that it works best with materials where the imaginary part of the complex refractive index is small, but not zero, compared to the real part of the complex refractive index. See, e.g. H. A. M. van den Berg and J. B. van Staden, Antireflection Coatings on Metal Layers for Photolithographic Purposes, J. Appl. Phys., 50 Mar. 3, 1979. This requirement narrows the list of desirable materials. While chromic oxide, Cr.sub.2 O.sub.3, has been used as an antireflection on some "darker" reflectors, such as Cr, Fe, and Ni (producing a standing wave ratio of about 0.4), it is not optimal, especially on "brighter" reflectors, such as Al (producing a standing wave ratio of almost 0.9). This is partly because Cr.sub.2 O.sub.3 has an imaginary refractive index equal to zero.
An alternative material and technique for producing an antireflective layer that did not depend on precisely matching the layer thickness to the exposure light wavelength would be a useful improvement. It would be further desirable if the improved technique for photolithography were easy to apply and produced fine lines.